Jupiter's magnetosphere, an ionized-gas bubble encasing
the planet, is lopsided and leaky, with an unexpected
abundance of high-energy particles bleeding out of one side,
according to recent measurements by NASA's Cassini spacecraft.

Those escaping electrons and ions might be riding
magnetic field lines that are attached to Jupiter at one end
and waving loose on the other, unlike more common lines that
loop between Jupiter's north and south hemispheres closer to
the planet.

Deciphering the process could advance understanding of
the protective magnetic field around Earth, as well as the
much greater one around Jupiter, said Dr. Dennis Matson,
Cassini project scientist at NASA's Jet Propulsion Laboratory,
Pasadena, Calif. Jupiter's magnetosphere is so vast that if
it shined at wavelengths visible to the eye, it would appear
from Earth to be two to three times wider than the disc of the
Sun, even though it is more than four times as far away.

"The dusk flank of Jupiter's magnetosphere is a
surprising contrast to the dawn flank," said Dr. Stamatios
(Tom) Krimigis, a Cassini scientist who heads the space
department of the Johns Hopkins University's Applied Physics
Laboratory, Laurel, Md. Cassini spent most of January and
February skating along the magnetosphere's dusk flank, which
is on the side of the planet turning away from the Sun. Other
spacecraft, such as Voyager, previously sampled the opposite
flank, corresponding to Jupiter's dawn side.

Cassini was flying past Jupiter last winter for a gravity
boost to reach Saturn. Researchers grabbed the opportunity to
study the giant planet from different vantage points by also
using NASA's Galileo spacecraft, which is orbiting Jupiter,
plus other spacecraft and ground-based telescopes, in
coordination with Cassini's Jupiter observations. More than 20
scientists are presenting some preliminary results from that
campaign during meetings of the American Geophysical Union in
Boston this week.

The electrons Cassini caught escaping may answer a
puzzle. Scientists had figured that some electrons were
getting out of Jupiter's magnetosphere, sometimes even
reaching Earth's neighborhood, but they didn't know the
primary route. "It appears we've found where they're coming
from," Krimigis said.

Dr. John Clarke of the University of Michigan, Ann Arbor,
used a movie taken by NASA's Hubble Space Telescope of
Jupiter's auroras while Cassini and Galileo were monitoring
Jupiter's magnetosphere and the solar wind, a flow of
particles speeding away from the Sun and deflected around the
magnetic fields of planets. Clarke said that movements of the
auroral glows indicate which features in them are linked to
the magnetosphere, because they follow the rotation of the
magnetic field, and which are linked to solar-wind effects,
because their positions stay oriented with respect to the
direction toward the Sun.

The timing and location of one patch of auroral
brightening captured by Hubble corresponded to a pulse of
electrons detected by Galileo in the magnetosphere. That pulse
appears to have been a type that also occurs in Earth's
magnetosphere, said Dr. Barry Mauk of Johns Hopkins
University's Applied Physics Lab, Laurel, Md., team member on
the energetic particle detector experiment on Galileo. "Energy
builds up in the system, pulling the magnetic field lines
outward like rubber bands, but eventually these rubber bands
can snap back toward the planet," Mauk said. The snapping back
brings an injection of high-energy electrons, he said.

Having Galileo inside Jupiter's magnetosphere at the same
time Cassini was just outside of it in the solar wind gave
scientists a chance to see whether such injections are
triggered by fluctuations in the solar wind, as can happen at
Earth. No obvious solar wind event corresponded to the
injections seen by Galileo. "It appears injections can happen
without being externally stimulated," Mauk said.

The solar wind does appear to have tipped features of
Jupiter's magnetosphere northward part of the time during the
Galileo and Cassini joint studies, said Dr. Margaret Kivelson
of the University of California, Los Angeles, principal
investigator for Galileo's magnetometer instrument. That gave
Galileo a taste of conditions that are usually farther south,
and it found that magnetic field lines there twist differently
than they do near the equatorial plane.

"It's as if a hula dancer had a skirt made of ribbons
that fly out as she twirls, but at one layer the ribbons twirl
in one direction and at a different layer they twirl in the
other direction," Kivelson said.

Jupiter's moon Io has its own auroras, which Cassini
captured in images taken while Io was in Jupiter's shadow. "We
could see that bright blue emissions near the equator move
around in a way that tells us their source," said Dr. Paul
Geissler of the University of Arizona, Tucson. The electron
flow causing gases to glow there comes from an electrical
current running between Io and Jupiter, he said. A new color
movie clip of the images is available at
http://www.jpl.nasa.gov/images/jupiter and at
http://ciclops.lpl.arizona.edu/ .

In addition, Io's volcanoes put out about a ton per
second of gases such as oxygen and sulfur. These are spun out
of Jupiter's magnetosphere and form a "Jovian nebula" that
extends tens of millions of kilometers or miles away from
Jupiter, Krimigis found with one of Cassini's sensors. "We
have even detected sulfur dioxide a long way from Jupiter," he
said.

More information about the joint Cassini and Galileo
studies of Jupiter is available at
http://www.jpl.nasa.gov/jupiterflyby . JPL, a division of the
California Institute of Technology in Pasadena, manages
Cassini and Galileo for NASA's Office of Space Science,
Washington, D.C. Cassini is a joint project of NASA and the
European Space Agency. The Space Telescope Science Institute
in Baltimore, Md., manages Hubble.